Ascot Gold Cup heroine Estimate, owned by Britain's Queen Elizabeth II, has tested positive for morphine.
Michael Stoute's five-year-old mare produced one of the stories of last season when triumphing in the two-and-a-half-mile feature Group One at Royal Ascot and went down on her sword when narrowly denied by Leading Light in defence of her crown last month.
The British Horseracing Authority announced last week that five horses under the care of various trainers have been reported to show the presence of morphine in their 'A' samples, a prohibited substance on raceday under the Rules.
Initial indications are that the positive test taken from Estimate resulted from the consumption of a contaminated feed product.
A statement from John Warren, the queen's bloodstock and racing advisor, said: "On Thursday July 17 the British Horseracing Authority (BHA) announced that a number of post-race samples, obtained from recent race meetings, had been found to indicate the presence of morphine, which is a prohibited substance on race days. Five horses, under the care of various trainers, were affected.
"I can confirm that one of those horses was Estimate, the five year-old filly trained by Sir Michael Stoute and owned by The Queen. Initial indications are that the positive test resulted from the consumption of a contaminated feed product.
"Sir Michael is working closely with the feed company involved to discover how the product may have become contaminated prior to delivery to his stables.
"As the BHA investigates this matter, including potential links between the different cases, Sir Michael continues to offer his full co-operation.
"There will be no further comment until the BHA announces its considered findings.
"Her Majesty has been informed of the situation."